Racetrack memory devices are gaining interest as high-density storage devices. These devices are disclosed, for example, in U.S. Pat. No. 6,834,005 titled “Shiftable magnetic register and method of using the same” issued Dec. 21, 2004 to Parkin, which is hereby incorporated by reference. FIG. 1 (which includes FIGS. 1A and 1B) illustrates an exemplary high-level architecture of such a racetrack magnetic memory system 100 comprising a magnetic shift register 10 that utilizes a writing device (also referred to herein as writing element) 15 and a reading device (also referred to herein as reading element) 20. Both the reading device 20 and the writing device 15 form a read/write element of system 100.
The magnetic shift register 10 comprises a track 11 made of ferromagnetic material. The track 11 can be magnetized in small sections, or domains, in one direction or another. Information is stored in regions such as domains 25, 30 in the track 11. The order parameter of the magnetic material from which the track is fabricated, which is the magnetization direction or the direction of the magnetic moment, changes from one direction to another. This variation in the direction of the magnetic moment forms the basis for storing information in the track 11.
In one embodiment, the magnetic shift register 10 comprises a data region 35 and a reservoir 40. The data region 35 comprises a contiguous set of domains such as domains 25, 30 that store data. Additional length is provided to the magnetic shift register 10 in the form of a reservoir 40.
The reservoir 40 is made sufficiently long so that it accommodates all the domains in the region 35 when these domains are moved completely from region 35 across the writing and reading elements (for the purposes of writing and reading domains) into region 40. At any given time, the domains are thus stored partially in region 35 and partially in region 40, so it is the combination of region 35 and region 40 that forms the storage element. In one embodiment, the reservoir 40 is where the reservoir region is devoid of magnetic domains in a quiescent state.
Thus, the storage region 35 at any given time may be located within a different portion of the magnetic shift register 10, and the reservoir 40 would be divided into two regions on either side of the storage region 35. Although the storage region 35 is one contiguous region, and in one embodiment of this application the spatial distribution and extent of the domains within the storage region 35 would be approximately the same no matter where the storage region 35 resides within the shift register 10, in another embodiment, portions of the storage region may be expanded during the motion of this region particularly across the reading and writing elements. A portion of (or even the entire) data region 35 is moved into the reservoir 40 to access data in specific domains.
The reservoir 40 is shown in FIG. 1 as approximately the same size as the data region 35. However, other alternative embodiments may allow the reservoir 40 to have a different size than the data region 35. As an example, the reservoir 40 could be much smaller than the data region 35 if more than one reading and writing element were used for each magnetic shift register. For example, if two reading and writing elements were used for one shift register and were disposed equally along the length of the data region, then the reservoir would only need to be approximately half as long as the data region.
An electric current 45 is applied to the track 11 to move the magnetic moments within domains 25, 30 along the track 11, past the reading device 20 or the writing device 15. The domains arrayed along the track are separated from one another by boundaries which are called domain walls (DWs). In a magnetic material with domain walls, a current passed across the domain walls moves the domain walls in the direction in or opposite to the current flow, depending on the characteristics of the magnetic material comprising the racetrack. As the current passes through a domain, it becomes “spin polarized”. When this spin polarized current passes through into the next domain across the intervening domain wall, it develops a spin torque. This spin torque moves the domain wall. Domain wall velocities can be very high, i.e., on the order of 100 m/sec or more, so that the process of moving a particular domain to the required position for the purposes of reading this domain or for changing its magnetic state by means of the writing element can be very short.
The domains, such as domains 25, 30, 31 are moved (or shifted) back and forth over the writing device 15 and reading device 20, in order to move the data region 35 in and out of the reservoir 40, as shown in FIG. 2 (which includes FIGS. 2A, 2B, and 2C). In the example of FIG. 2A, the data region 35 could initially reside on the left side of the well, i.e., bottom section 32 of the magnetic shift register 10, with no domains in the reservoir 40. FIG. 2C shows the case where the data region 35 resides entirely on the right side of the magnetic shift register 10.
In order to write data in a specific domain, such as domain 31, a current 45 is applied to the magnetic shift register 10 to move domain 31 over, and in alignment with, the writing device 15. All the domains in the data region 35 move when the current is applied to the magnetic shift register.
The movement of the domains is controlled by both the magnitude and direction of the current, and the time over which the current is applied. In one embodiment, one current pulse of a specified shape (magnitude versus time) and duration is applied to move the domains in the storage region in one increment or step. A series of pulses are applied to move the domains the required number of increments or steps. Thus, a shifted portion 205 (FIG. 2B) of the data region 35 is pushed (shifted or moved) into the reservoir region 40. The direction of motion of the domains within the track 11 depends on the direction of the applied current.
In order to read data in a specific domain, such as domain 25, additional current is applied to the magnetic shift register 10 to move domain 25 over, and in alignment with, the reading device 20. A larger shifted portion 210 of the data region 35 is pushed (shifted or moved) into the reservoir 40.
The reading and writing devices shown in FIGS. 1 and 2 form part of a control circuit that defines a reference plane in which the reading and writing devices are arrayed. In one embodiment, the magnetic shift register 10 stands vertically out of this reference plane, largely orthogonal to this plane.
In order to operate the magnetic shift register 10 the control circuit includes, in addition to the reading and writing elements, logic and other circuitry for a variety of purposes, including the operation of the reading and writing devices, the provision of current pulses to move the domains within the shift register, the means of coding and decoding data in the magnetic shift register, etc. In one embodiment, the control circuit is fabricated using CMOS processes on a silicon wafer. The magnetic shift registers will be designed to have a small footprint on the silicon wafer, so as to maximize the storage capacity of the memory device while utilizing the smallest area of silicon to keep the lowest possible cost.
In the embodiment shown in FIG. 1, the footprint of the shift register will be determined largely by the area of the wafer that the reading and writing devices occupy. Thus, the magnetic shift register will be comprised of tracks extending largely in the direction out of the plane of the wafer. The length of the tracks in the vertical direction will determine the storage capacity of the shift register. Since the vertical extent can be much greater than the extent of the track in the horizontal direction, hundreds of magnetic bits can be stored in the shift register yet the area occupied by the shift register in the horizontal plane is very small. Thus, the shift register can store many more bits for the same area of silicon wafer as compared to conventional solid state memories.
Although the tracks of the magnetic shift register are shown as being largely orthogonal (i.e., “vertical”) to the plane of the reading and writing elements (the circuitry plane) these tracks can also be inclined, at an angle, to this reference plane, as an example, for the purpose of greater density or for ease of fabrication of these devices. The tracks can even be parallel to the surface of the reference plane, i.e., they may have a “horizontal” configuration.
In an entirely uniform, smooth and magnetically homogeneous track the energy of a domain wall does not depend on its position along the track and thus a domain wall may not be stable against thermal fluctuations and parasitic magnetic fields, e.g., those from nearby domain walls in the same or adjacent tracks. By varying the structure of the racetrack (see U.S. Pat. No. 6,834,005 to Parkin), preferred positions for the domain walls along the track can be formed where the energy of the domain wall is decreased. These “pinning sites” thus play an important role in defining energetically stable positions for the DWs and thereby the spacing between consecutive bits.